1. Field of the Invention
This invention relates to a fluid monitoring apparatus for detecting the presence of and determining the characteristics of particulate matter suspended in a fluid. More specifically, this invention relates to a device for the real time identification of the size and shape of a particle within a fluid by forming an optical image of the fluid and analyzing the image.
2. Description of the Related Art
Determination of the quantity, characteristics, and types of particulate matter in fluids is important for many applications such as monitoring fluids in engines and rotating machinery, industrial quality control, food processing, medical analysis, and observing environmental controls. Current devices for monitoring particulate matter in fluids involve various mechanical, electrical, or optical means.
Purely mechanical means involve collecting particulate matter in suitable traps such as filters, screens, or magnetic plugs. Analysis of the particles is done by removing the traps and examining the collected matter. These devices do not provide for real time monitoring. Electrical based particle monitoring devices are based upon induced currents. By sensing or measuring the change in an electric current, the concentration of electrically conducting or nonconducting particles can be determined. However, this technique is not able to identify the size or shape of the particles. In conventional electrical-mechanical devices, such as electric chip detectors in engines, only electrically conducting particles are detected and the detectors provide no identification on the size or shape of the particles.
Optical devices have been used to determine the concentration, number, or size of particles. However, these devices are limited in their ability to identify the shape of particles. They identify the shape of particles by differentiation based on the degree of sphericity (or asymmetry) of the particles. Such devices illuminate a fluid sample and measure the intensity of the light scattered and/or transmitted at various detectors surrounding the sample. By comparing the intensity measured at the various detectors, a degree of sphericity or an asymmetry factor for the particle is determined. The particles are grouped by this degree of asphericity rather than identified or classified by the shape of the particle.
Similarly, the size of the particles is obtained by comparing the average intensity with a reference intensity or with the difference in intensity for a number of particles with a similar asphericity over time. This method yields information on the size of particles but does not provide real time size and shape identification.
Additionally, optical devices are subject to operating requirements that limit the number and type of applications for which they can be used. One such requirement is that the particles be passed one at a time or that the fluid be passed in a small volume through a specific point because the light illuminating the particles is directed to that specific point. Additionally, in monitors that simply measure the light scattered from a single particle, the determination of symmetry is based upon the distribution of light scattered from a single particle. In such monitors, the symmetry of a particle cannot be accurately determined if two or more particles are illuminated at the same time. These requirements limit the application to those in which either the position of the particle is known and controlled or only a small sample of the fluid volume is passed to a monitor.
Another limitation of optical devices results from their sensitivity to air bubbles within the fluid. Entrained air bubbles are only distinguished from other particles by the high degree of sphericity or symmetry of the air bubbles. To remedy this, some devices must be oriented so that the air bubbles float to the top or upper area of the chamber containing the sample and, therefore, are removed from the sensing area of the fluid. Other devices will account for air bubbles by ignoring all highly spherical particles in any analysis of particulate matter.
Real time human visual inspection of particles can be accomplished by providing a window through which an individual can view the fluid. However, this method is limited to fluids flowing at low rates of speed. While using a strobe lamp or pulsed light to create the visual effect of stopping the motion of the particles will allow visual inspections for fluids flowing at higher speeds, these increased speeds are far below those encountered in many applications, for example, the speed at which oil flows in aircraft engines. Additionally, visual inspection is limited to those applications in which the particles are of sufficient size to be viewed by an individual and to those applications in which the fluid is sufficiently transmissive to light of wavelengths which can be seen by humans.
An optical device which forms an image of the fluid would be advantageous because it would be able to combine the advantages of visual inspection with the advantages of optical devices. More specifically, by analyzing the image formed, an accurate and real time determination of the size and shape of particles within the fluid can be accomplished. Additionally, devices which are sensitive to a wide range of particle sizes and wavelengths of light can be made.
The need for a device and method which can identify the size and shape of particulate matter within a fluid becomes readily apparent when one considers the problem of monitoring for particulate debris in the oil of helicopter or other aircraft. Particulate debris in helicopter or other aircraft engines can cause engine failure and loss of life. As a helicopter engine ages, particles from the engine or gear box components (bearings and gears) tend to flake off into the engine's oil. The size, shape, and density of flakes, as well as other types of debris, in the oil are indicative of engine condition, and can indicate when engine failure is imminent. Currently, magnetic plugs are removed for visual inspection of debris to spot upcoming engine failure. However, this must be done frequently to ensure that no failures occur. This routine maintenance is costly, time consuming and often unnecessary. An oil debris monitoring system that provides real time information on the size and shape of particles within the oil will provide an early warning system for engine failure and can be used to record engine condition to identify when maintenance is needed. Thus, a system that can monitor size, shape, and density of particulate debris in engine oil in real time would be a welcome addition to the art.